This invention relates to data communication and, more particularly, to a method for improving successful reception of binary messages without increasing overhead as compared with using a retry scheme for incorrectly received messages.
Data communication may be required from a plurality of physically dispersed remote stations to a central receiver. The communication medium may be situated in a hostile environment, such as power line carrier or factory communication system, which typically experiences randomly varying electrical characteristics, such as noise. Environmental factors affecting electrical characteristics may include lightning and operation of heavy machinery. Typically, transmissions from remote stations arrive at the central receiver with an acceptable signal-to-noise (S/N) ratio (i.e. low probability of bit error or less than about 0.0005). However, due to electrical path characteristics, transmissions from some remote stations may arrive at the central receiver with marginal S/N (i.e. high probability of bit error or greater than about 0.005). Additionally, data may be corrupted during travel along the transmission medium by momentary changes in electrical characteristics of the transmission path. Increasing the power of the transmitted data from the remote station in order to increase the S/N ratio is not always practicable, since an increase in power generally increases the cost of the transmitter, and for a system having many remote stations the increased cost may be infeasible. Also, it is generally desirable to employ the same transmission protocol for all remote stations in the system.
A typical scheme for data transmission over a communication medium in a master (i.e. requests data transmission)/slave (i.e. sends data in response to request) environment uses error detection at the master to determine the integrity of the message received from the slave. Alternatively, remote stations may transmit at predetermined intervals or when they have message words available wherein the central station requests a retry or retransmission only when a message word is detected as containing error. If an error is detected in the message by the master then an automatic repeat request (ARQ) may be sent from the master to the slave. Upon receipt of the ARQ, the slave retransmits the same message which had the error as detected by the master. With a high S/N data path, message retransmission or a variation thereof such as is described in chapter 15 of "Error Control Coding: Fundamentals and Applications" by Shu Lin and Daniel J. Costello, Jr. (1983) (hereinafter Lin and Costello), when an error is detected, offer an efficient strategy. Error detection may be implemented by any known coding technique, such as CRC16 or CRC32 (CRC=cyclic redundancy check) at the slave, with corresponding decoding at the master. These techniques typically involve concatenating a data word, i.e. intelligence, with an error detecting parity word at the slave to form a message word which is transmitted to the master.
For low (S/N) data paths over the transmission medium, such as may be experienced in power line carrier or factory environment communication systems, the probability of bit error may be relatively high (e.g. greater than about 0.005). In such cases, the probability of obtaining an error free message (given that the message first received included an error) using a single retry is very low. For example, if the probability of bit error is 0.02 and the message length including the CRC check is 228 bits, computations indicate that the probability of success using a single retry would be about 1.988 percent. It is generally desirable to employ the same message word transmission protocol for all remote stations in the system and further it is not usually known in advance of message word transmission which data paths may experience a degradation in electrical path quality.
Error correction codes, which are transmitted with the message, have been used on fading and other low S/N paths. However, generally these codes significantly increase the traffic rate, resulting in lower system transmission efficiency and are subject to correcting to a false message (i.e. message detected as having no errors by the master but not the message sent by the slave) when used in an environment subject to high bit error rates.
Accordingly, it is an object of the present invention to increase probability of successful message reception without increasing message traffic above that used with a conventional retry scheme.
Another object is to reduce the probability of correcting to false message.
Still another object is to increase the efficiency of a communication system having data transmission signal paths with randomly varying electrical characteristics.